1. Field of the Invention
The present invention relates generally to gas turbine engine compressor outlet guide vanes and diffuser assemblies and, more specifically, to aerodynamically efficient vanes of the assembly.
2. Background Art
A conventional gas turbine engine includes in serial flow communication a compressor, a discharge flowpath having a stage of compressor outlet guide vanes (OGVs), disposed between annular inner and outer walls, which in turn are mounted in an OGV support structure mechanically tied into an engine casing. Outlet guide vanes typically have airfoil like cross-sections that include a leading edge, a relatively thick middle section, and a thin trailing edge. Downstream of the OGVs is a combustor diffuser, a combustor, a turbine nozzle, and a high pressure turbine. Typically, OGV inner and outer walls are supported by corresponding inner and outer annular diffuser inlet walls to form a relatively leak-free flowpath therebetween and support the OGVs and diffuser. The OGVs, inner and outer walls, and diffuser may be a single piece, integrally cast assembly or in some other constructions corresponding inner and outer OGV walls with the OGVs therebetween are welded to a diffuser casing.
During engine operation, the compressor compresses inlet airflow, which is therefore heated thereby. The discharged compressed and heated airflow is then channeled through the OGVs and the diffuser to the combustor wherein it is conventionally mixed with fuel and ignited to form combustion gases. The combustion gases are channeled through the turbine nozzle to the high pressure turbine which extracts energy therefrom for rotating and powering the compressor.
Typically, the high pressure air at the compressor exit is conditioned to have low swirl and low Mach number for use in the combustor and the outlet guide vanes and diffuser are employed to condition the compressor discharge air to be suitable for the combustor. Some engine configurations also require the OGVs to serve as a structural member which places additional constraints on the design. Conventionally, outlet guide vanes reside in a constant annulus height flowpath. The flowpath may help turn the flow radially outwardly to help align it with the downstream combustor. The OGVs are designed to remove tangential swirl from the compressor discharge air so that upon leaving the OGVs air flows nominally in the axial direction. In the process of deswirling, the flow""s tangential momentum is converted to static pressure, reducing the flow""s absolute Mach number. The diffuser is defined as the flowpath section downstream of the OGV trailing edge, which further decreases the flow Mach number by one or by a plurality of divergent annular passages. These passages may also guide the flow radially outwardly, providing yet more diffusion for a given annulus height. Adequate efficiency and stall margin are obtained by employing sufficient airfoil solidity, selecting proper airfoil incidence, optimizing the surface velocity distributions, and providing enough diffuser length/area ratio to avoid flow separation. High efficiency and reduced length typically requires reduced airfoil solidity and diffuser length to reduce wetted area and, therefore, reduce drag. For a given static pressure rise requirement, this loads the surface boundary layers bringing them closer to separation.
It is desirable to supply high pressure compressor exit air to the combustor as efficiently as possible with sufficient stall margin while minimizing engine length and hence weight and cost. Reduced length typically results in higher diffusion rates which makes the boundary layers more susceptible to separation which negatively impact performance and stall margin. Thus, reduced length and low diffusion rates tend to be conflicting requirements. In order to gain a competitive advantage it is desirable to reduce the axial length required to deliver this air and hence to reduce engine length, weight, and cost while maintaining performance and stall margin.
A gas turbine engine outlet guide vane assembly has annular inner and outer end walls, a flowpath between the inner and outer end walls, outlet guide vanes radially disposed between the inner and outer end walls, and a boundary layer energizing means for energizing boundary layers using secondary flow to mix free stream flow into the boundary layers along the inner and outer end walls and suction and pressure sides of the vanes. Secondary flow is any flow not in a direction of the primary flow. Free stream flow is any flow outside of the boundary layers. Secondary flow and primary flow are discussed in great detail in an article entitled xe2x80x9cSpanwise Mixing in Axial-Flow Turbomachinesxe2x80x9d by Adkins and Smith in the January 1982 volume of the Journal of Engineering for Power, pages 104-110. The vanes have pressure and suction sides and a first boundary layer energizing means includes the vanes being circumferentially leaned in a circumferential direction that the suction sides face. A second boundary layer energizing means includes swept leading and/or trailing edges of the vanes which extend radially between the inner and outer end walls. In a more particular embodiment of the invention, the swept leading and/or trailing edges the are curved inwardly into the vanes from the outer end walls to leading and trailing edge points, respectively, that are located between the end walls. A third boundary layer energizing means includes the vanes being bowed circumferentially outwardly such as in a circumferential direction the pressure side is facing. The exemplary embodiment of the invention incorporates all of these boundary layer energizing means. The invention also includes a diverging flowpath between said leading and trailing edges.
The outlet guide vane assembly may be used in a gas turbine engine outlet guide vane and diffuser assembly having integral outlet guide vane and diffuser sections which share common annular inner and outer end walls radially bounding the sections and the flowpath between the inner and outer end walls. The outlet guide vane section is located forward of the diffuser section and includes the outlet guide vane assembly with the outlet guide vanes radially disposed between the inner and outer end walls. The boundary layer energizing means enhances secondary flow mixing of boundary layers along the inner and outer end walls and the suction and pressure sides of the vanes. The diffuser section can include struts extending radially across the flowpath between the inner and outer end walls in the diffuser section and/or annular flow separators.
The invention provides a design that reduces the axial length of the outlet guide vane and diffuser assembly used to deliver compressor air to a combustor which has been deswirled and diffused. The invention reduces engine length, weight, and cost while maintaining acceptable levels of engine performance and stall margin.